I'm trying to install a add-on (r.hydrodem) for GRASS7, but encountered some problems doing this. After correspondence with the author of the add-on, it turns out that my version of GRASS, r61585M, is causing the problems. I need at least revision 62324 for the add-on to work.

Is it possible to update to this revision on a Mac? And if it is, how?

I just posted new binaries a few days ago. This may solve your problem. Be sure to update to the newest GDAL (on the frameworks page). A warning, there may be a problem with the most recent GRASS 6.4 binary. Don't overwrite your current version until you see if the new one displays maps properly. I have kept the older GRASS 6.4 svn up just in case.